1. Field of the Invention
The present invention is related to a receiver for a wireless communication system, and more particularly, to an interference-robust receiver which provides highly linear baseband signal for a wireless communication system.
2. Description of the Prior Art
An important concern when designing radio-frequency (RF) receivers for wireless communication systems is to detect a very weak in-band signal in the presence of a strong out-of-band jammer. If the linearity of the receiver is not good enough, such jammer may saturate the receiver and block the in-band signal. Using a surface acoustic wave (SAW) filter in front of the receiver is the most common solution to this problem. The SAW filter has band-pass capability with a very high quality (Q) factor, thereby capable of providing a large rejection ratio to the out-of-band jammer (normally greater than 20 dB) and meeting the receiver linearity requirement.
FIG. 1 is a functional diagram illustrating an exemplary prior art receiver 100 for a wireless communication system. The receiver 100 includes a SAW filter 102, an RF signal processor 110, a frequency conversion interface 120, and an analog signal processor 130. The SAW filter 102 is a frequency-selective device which passes in-band part and attenuates out-of-band part of the received RF signal. The RF signal processor 110 includes a matching network 112 for power matching or noise matching and a low noise amplifier (LNA) 114 for signal enhancement. The prior art frequency conversion interface 120 adopts a mixer 126 which operates according to a local oscillator (LO) signal. After signal filtering and amplification, the RF signal is then down-converted to an intermediate frequency signal by the mixer 126. The analog signal processor 130 can thus process the intermediate frequency signal for subsequent applications.
There are several drawbacks associated with the prior art receiver 100. The first is that in-band attenuation tends to make it harder to detect weak signals, creating the need for an even more sensitive receiver after the SAW filter. More importantly, there is currently no economical way to implement SAW filters or their equivalents in the same processes as the active circuits that follow them, which are typically produced using CMOS or BiCMOS processes and either silicon or silicon germanium technologies. The result is that SAW filters significantly increase the cost and consume equally valuable circuit board area in a typical communication device. This problem is further exacerbated by the proliferation of different frequency bands that a communication device has to be compatible with.